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Over the past year, I’ve heard stories of people who made custom Kaizen foam inserts to organize their tools have their hard work melted by leaving their box in the sun.

I just recently saw another person show their melted foam insert and I started thinking:

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Why does this only seem to happen with the clear-lidded organizers?

How hot does it get inside these organizers?

Is the foam itself contributing to the problem?

This isn’t just limited to foam inserts. People store cordless power tool batteries and other temperature-sensitive items in these clear-lidded cases. For example, Milwaukee sells promotional M12 cordless tool kits, and packages them in the clear Packout cases.

What might happen to a lithium ion battery at the temperature foam melts, after cycling through high temperatures over a long periods of time?

Both Stuart and I ran a few tests and got some pretty interesting answers to the above questions and like any good investigation — more questions. But first let me take you through the theory, feel free to skip to the “testing” section if you just want to see the results.

Theory

I thought the answer to the first question — why does this only seem to happen with the clear-lidded organizers? — was obvious: the case is acting like a greenhouse. You don’t just see this effect in greenhouses; think about a your vehicle on a sunny day, a solar hot water collector, or if you are a gardener, a cold frame for extending the growing season.

Left: Hand above Packout clear lid, Right: Hand below lid

There are a few principles at work here:

The clear lid is transparent to visible (and possibly ultraviolet) light, and opaque (not transparent) to infrared light, while a solid lid is opaque to both visible and infrared

Materials can be heated by absorbing visible light and re-emit some of that energy as infrared light

Heat is transferred by conduction, convection, and radiation

Putting it all together: the clear lid is allowing the visible light to pass through the lid, and that light is absorbed by the material inside heating it. The material re-emits some of this energy as infrared light that can’t pass through the clear lid, and so it bounces around until it is absorbed again, further warming the interior of the box. Since the container is weatherproof, and only holds a small volume of air, convection is very limited. The only way for heat to escape is to be conducted through the material of the box.

In reality, of course things aren’t this simple, but we believe it’s a good first-order explanation of what is happening.

Why do the clear-lidded boxes get hotter? Because the clear lid allows more energy into the box than a solid lid would, and most of that energy has no quick way out. The solid lid can only allow heat into the box by absorbing the sun light and conducting it through to the inside of the box.

After some research online, I found a different possible answer. One company that sells Kaizen foam inserts for these organizer boxes — Kaizen Inserts — has the following warning on one of their pages:

NOTE/WARNING: This box has a clear lid, keep out of sunlight, as the lid acts as a magnifier and will melt the foam!

So Stuart and I designed some tests that would would test each theory.

Preliminary Testing

All my testing was done on or around the deck in my front yard. It faces south and I have trees on both side, so it only gets sun between 11 am and about 2:30 pm. The ambient temperature on all the days was in the 80 to 90°F range, and it was partly cloudy.

The first test I ran was to see how hot it got inside an empty clear-lidded Packout organizer. I was getting peak temperatures above 140°F.

This leads to the question: how do we know it’s the clear lid? Stuart came up with an excellent test – what happens if you perform the experiment with the box upside down?

In this photo, you’ll also notice I have the meter in the shade. I found out pretty quickly in the first test that the meter started reading higher if it was in the sun. Even though I have it in the sun in the first photo, it was moved into the shade when I was taking data.

Here’s the data from the two tests. You can see that with the clear lid face up, the temperature varies up and down quite a bit, I would posit the temperature is following the cloud cover. Every time a cloud blocks the sun, the temp starts falling, when the sun comes out again the temperature rises again.

Notice how smooth the temperature data is inside the case when the clear lid is face down. It isn’t varying much with cloud cover. It slowly rises, levels off, and then falls as the day gets longer.

To be fair, I also tested a Dewalt clear-lidded organizer too. I left the yellow bins inside because the case is black. I was afraid that would skew the results of the test. Having the bins in place will also cut down on the amount of convection, or at least localize it to each bin.

This Dewalt organizer data doesn’t show as much of a clear difference between the clear lid being face up or down. The effect still seems to be there, with the interior of the organizer getting hotter when the clear lid is facing up.

I’m thinking the bins might have complicated the experiment by adding more thermal inertia — the additional plastic holds more heat and is more resistant to temperature fluctuations. Plus it limited air movement to little pockets.

The internal temperature with the black back of the case facing up rose higher than with the red Packout, possibly because the black plastic absorbs more of the sun’s radiation more effectively than red.

Melting Foam

I put some test scraps of foam in the clear-lidded Packout and set it in the sun. I saw melting foam within half an hour, but they weren’t big enough to resolve whether the melting was due to lensing or the additional trapped heat. If the melting was due to lensing there would be more melting under the angled areas than the flat areas.

If my hypothesis was correct, there would be more melting where the sun went through the clear areas and further raised the temperature of the foam vs the frosted areas.

At this point I cut out a Kaizen foam insert to fully cover the bottom of the case so I could see the pattern of melting and then logged the temperature as I left it in the sun.

Wow! Lining the Packout tool case with a foam insert caused the temperature inside to reach nearly 200° F. You’ll notice that I left the tool box outside for 3+ hours for this test, in hopes of getting a good melting pattern. Those really sharp dips in temperature correspond to me opening the case to take photos of the foam. You can see that the temperature quickly rises again when I shut the case.

There was visible melting before I checked the case the first time, meaning that the foam can start melting in direct sunlight in under 30 minutes.

I left the foam melt some more and after a while I think it’s pretty clear that the foam is just melting under the clear sections of the lid and not the frosted.

I let the experiment run as long as I had direct sunlight, which resulted in the Hero image for this post. Due to the changing angle of the sun, the limited run better shows the pattern (although the full run better shows the extent of the damage).

The last thing I wanted to do was make sure the foam only melts due to the greenhouse-like environment created inside the box, and not just because the lid magnifies the sun. So I grabbed two new pieces new foam and ran another experiment.

I left one piece of foam exposed directly to the sun and the other under just the clear lid with no enclosure. I balanced the lid on top of some of the organizer bins to approximate the distance of the lid from the foam in the box and left the experiment out in the sun. I did not measure the temperature this time.

After 3+ hours of exposure neither piece of foam showed any signs of melting.

Conclusion

I would not call any of the testing we performed a rigorous scientific study. We did not carefully design the procedure and eliminate every conceivable variable. Nor did we run enough trials to get statistically significant data either. But I wouldn’t dismiss our results out of hand either, and I think our tests demonstrate some neat physical principles.

In the beginning of the post I asked three questions:

Why does this only seem to happen with the clear-lidded organizers?

How hot does it get inside these organizers?

Is the foam itself contributing to the problem?

Why does this only seem to happen with the clear-lidded organizers? I hypothesized that it was due to the box becoming a greenhouse. I think the data supports that, but it isn’t totally conclusive.

How hot does it get inside these organizers? Pretty damn hot. I measured temperatures of nearly 200°F inside a clear-lidded Packout organizer with a Kaizen foam insert that was left face up in the sun.

Is the foam itself contributing to the problem? I think the data points to yes. The internal temperature of the Packout organizer was much higher when there was a foam insert inside the box.

Ben’s Final Thoughts

As with any good experiment, we were left with more questions than answers. For one, the black foam definitely exaggerated the issue. Would a different color foam (maybe white foam), or a different brand or type of foam fare better?

Does the color of the organizer itself matter? It might be interesting to see how different organizers, such as Dewalt’s, compare to Milwaukee’s, when empty or lined with Kaizen foam inserts.

What would happen if I put new foam in the case and placed it in the sun upside down. Are there any inherent properties of the foam that cause the temperature to rise?

Additional Testing

(Stuart joins in the fun. Here’s what he did, in his words.)

Ben gathered some excellent data, and I couldn’t let him have all the fun.

Milwaukee had sent me a Carbide Challenge kit (thank you!), complete with a custom-cut foam insert and a selection of linear cutting tools.

(Why haven’t you heard about this kit sooner? Well, I am extremely fond of Milwaukee’s hole saws, Sawzall blades, and carbide-tipped Sawzall blades. I already know they’re excellent, and with a full workload I have not yet been able to put them through extreme torture tests.)

I thought this would be an excellent way to change up the testing parameters.

Compared to Ben’s testing, my tests involved a different type of foam, as well as mixed contents. Contents include 2 different types of hole saws, Sawzall blades in their own clear-lid container, and hole saw arbors.

Setup & Test Equipment

Test equipment included:

Fluke T3000 thermometer with K-type thermocouple

Fluke E4 thermal imaging camera

Milwaukee cooler for protecting the Fluke meter from direct sunlight and the heated environment

The thermocouple was placed slightly beneath the top surface of the foam, embedded between one of the white hole saws and the foam surrounding it, about 1/3 of the way from the front and side walls.

A small section of firm elastic cord was placed between the lid and bottom of the Packout box, so as to protect the thermocouple wire from excessive pressure and strain. This created a very slight gap that I determined to be negligible.

My test was done under mid-afternoon sun (~3:45pm), and with recorded temperature of 97°F. My weather report app told me that it “felt like” 104°F.

Just after starting the test, the meter read 103.5°F, while it measured just under 90°F itself.

Some time into testing, the meter read 121.1°F, while thermal imaging recorded a meter housing temperature of 94°F.

My thermal imaging camera was not optimized for outdoor conditions, and so its readings shouldn’t be strongly considered for their quantitative value, but for comparison purposes.

At the end of testing, the meter housing had measured close to 121°F. It increased in temperature during the course of testing, but at a much slower rate than if it were placed on the bare ground or left in the sun. The equipment could be better insulated, but placing it within an empty cooler with the top down but unzipped was better than nothing.

Data

At the start, the top lid quickly jumped up a few degrees.

At the end of the test, the driveway temperatures were a touch cooler, but not significantly.

The middle of the Packout lid measured around 133°F. This was quite a bit hotter than ambient temperatures, but lower than the driveway pavement. It was also considerably cooler than my car.

(The time stamp is inaccurate.)

Here’s a look at a couple of areas of interest. The outside of the Packout lid varied in temperature by quite a bit; there was a nearly 9°F difference between opposite sides of the lid (top and bottom in the image, or left and right with respect to the handle).

Remember – since the clear lid is opaque to infrared light, the thermal image only shows the surface temperature, and not any of the temperatures underneath.

At location 3 (spot measurement 3 or SP3), there is a 16°F bump-up in temperature. At this location, the lid is recessed and brought in closer proximity to the black grommet weather seal, as well as a raised section of the lower tool box.

Measurements 4, 5, and 6 were taken of the driveway. There is some variation between spots 5 and 6, both still in full sunlight. But location 4, which was in the hotter part of the shadow cast by the Packout tool box, was significantly cooler than surrounding areas.

As we established, the outside of the Packout tool box had reached a temperature of around 133°F. The external lid temperature was not constantly monitored.

Temperature of the foam inside the tool box.

As mentioned, the thermocouple temperature probe was placed between a hole saw’s outer wall and the fitted foam material surrounding it. As such, the temperature measurement was not quite of the surface of the foam.

The temperature increased quite rapidly. At 10 minutes in, the temperature had increased to 110°F. After 40 minutes in the sun, the temperature had reached nearly 156°.

The maximum temperature, 163.3°, was reached after 63 minutes. From there, the temperature fluctuated a little, and looking at the data, the average temperature started to decrease after a couple of minutes. At the 71 minute mark, the average temperature had begun to drop by around 0.1°F/minute.

Happy that the case contents seemed to have reached thermal equilibrium, I ended the test and wrapped up with thermal imaging.

Once more, here is a look at the Packout tool box at the end of the test, just prior to opening the lid. As you can see, there are some hot spots, especially in the area closer to the front (handle-side) of the case. One side of the lid measures cooler, indicated by the darker blue coloration.

Opening the case, we see an uneven heat distribution. Thermal imaging gives us a spot measurement of 175.2°F, just near where the thermocouple is placed. The maximum recorded thermocouple temperature was 163.3°F.

Here’s a reminder of what’s in the case.

As for the foam – there was NO melting. It felt a maybe little softer, but there was no visible deformation. Once the Packout case had been given sufficient time to cool down indoors, the foam appeared to be perfectly intact.

According to the timestamp, the case had been open for less than one minute, and there was still a hot-spot measuring 174.4°F. Where the thermcouple was still embedded, the thermal imager recorded a temperature of 160.8°.

Here is a tough question. Is one side of the case hotter and the other side cooler because of the thermocouple wire breaking the perfect seal? I don’t think so, but it is something to consider.

A plastic case of Sawzall reciprocating saw blades is on the cooler side of the Packout tool box.

From the thermal image, we would think that the Sawzall blade case was significantly cooler than the rest of the case. And while true, the temperature inside the case is different.

Looking at the blades inside, thermal imaging gives us a temperature measurement of 150.9°F.

Stuart’s Thoughts and Conclusions

One thing is for certain – the contents of clear-lidded Packout organizers get very hot when left out in the sun, and it happens quickly.

In my testing, which started after 3pm, the sun was not directly overhead, as you can see from the size of the planter shadow in the last image. I am certain that direct sunlight would have heated up the Packout case contents to even higher temperatures.

Keep in mind that my testing also focused on the Milwaukee Packout organizer, but we believe that all clear-lidded organizers might behave in a similar manner.

The internal organizer temperatures we both observed in our testing are excessively high for certain tools and supplies. You definitely don’t want something like a cordless power tool battery pack in a clear-lid organizer that’s in your truck bed and exposed to the sun.

Why is this happening? As Ben mentioned, the black foam is absorbing much of the incident visible light, and reflecting little of it. That high intensity visible light is converted into thermal energy, which becomes trapped within the case.

There is not a lot of air volume within a case, and with a good seal, there’s negligible convection. Convection will help keep cool an organizer’s external surfaces, however. Conduction isn’t going to play a big part in cooling down internal components.

We are not quite certain as to how much of the absorbed visible light is converted to infrared light via radiation, but any such light will be trapped within the case, and be absorbed or reflected by anything it comes into contact with. Light can be transmitted, absorbed, or reflected, and as discussed earlier, the organizer lid is shown by the thermal imaging cameras to not transmit IR wavelengths.

At the least, exposing a clear-lidded organizer to direct sunlight will increase the temperature of whatever’s inside. Sometimes it might not matter. Other times it could pose an inconvenience. Who wants to touch hot screws or nails? A hot battery won’t let itself be recharged. Certain materials degrade at elevated temperatures, such as certain types of layered foam used to make custom inserts (but not the stuff Milwaukee used for their Carbide Challenge media kit).

At worst… well, use your imagination.

From what I’ve seen, there is one clear solution. Don’t leave clear-lidded organizers in the sun. If it’s at the top of your Packout or ToughSystem tool stack and secured in your truck, toss a cheap white towel over them. Do whatever it takes to keep them out of direct sunlight. Hypothetically, you can place a white insert into the lid of the organizer, but that defeats the purpose of the clear lid, doesn’t it.

Finally, keep in mind that our testing involved organizers fitted with dark-colored inserts meant to be custom-fit for tools. This creates worst-case conditions. If your organizer is equipped with red, yellow, or orange bins, and filled with rivets, screws, or other conductive materials , you’re probably not going to see temperatures anywhere close to what we measured.

Hmm, I wonder what would happen if a white foam insert is used instead of black…

A lighter or white-colored foam might reduce the average temperature inside a clear-lidded organizer or tool box, but black or dark-colored materials, such as battery packs, will still absorb visible light and heat up.

Environmental Images

The garbage can lid was hot, measuring 150°F at the center.

The hood of my dark blue car was extremely hot, measuring 190°F at the part directly aimed at the sun.

The driveway measured 154.4°, with hotter and cooler areas.

It’s hard to imagine it, but the driveway measured as cool as 101°F where a small planter cast its shadow. Surrounding areas exceeded 150°F.

About Benjamen

Benjamen Johnson grew up watching his dad work as a contractor and woodworker. He became an electrical engineer and took an interest in woodworking. Check out Ben's projects at Ben's Workshop or follow him on twitter or instagram.

So based on your temperature measurements and combine that with the thin black surfaces of the “open-cell” foam (the bubble walls) allows for that heat to easily be absorbed. The foam may not be melting but it should definitely be collapsing. Which your images show.

Some of the best tests are actual real world tests, and this one showed great illustration.

The thing with outside environment tests is that you cannot control the weather, nothing is ever consistent or repeatable. So the best thing you can do is take a bunch of data, which is what you guys did. Lab tests are important, but tests like this can be way more valuable.

It is good to keep in mind that any numerical infrared temperature measurements are not necessarily accurate. The images are really good for showing relationships and comparisons to the surrounding surfaces.

Also, apparently TC’s can become inaccurate too as the TC gets older. However this is usually fairly small. I’m glad you guys used TC measurements because they do produce very accurate readings.

Interesting, I never thought there was anything more complex than the foam melting, but you are probably right, much of the damage looks like collapsing.

But there definitely is some limited melting going on too — there are places where there are hard pieces like melted plastic. I’ve melted kaizen foam with a hot copper pipe to make finger holes, and some of the damage is consistent with that.

Possibly the glue is affecting the results too? Kazien foam is thin layers of foam glued together. We don’t know what the properties of the glue are.

I was trying to find out more info on what FastCap uses to glue the sheets. They may be using their own 2P-10 or nothing other than the foam itself. They use this glue in one of their videos describing how to glue two Kaizen peices together.

Also, it looks like it is closed-cell foam, I was wrong above. But now I can say they are little green houses.

Their 2P-10 appears to be similar to superglue (Ethyl-2-cyanoacrylate). From some very quick search. It appears that these glues break down at around the same temps as the foam, which just so happens to be similar temps you measured.

Awesome writeup, this is great original material. As mentioned, the takeaway is: Don’t leave your gear in the sun! Going one step further, I’d be interested in seeing how many extended heat cycles occur at those temps before we see failure or compromise of windings, leaking lube from bearings/gears, failing batteries, etc. That’s much more difficult to test…but it can’t be good!

I only read a paragraph or two of the article, but I can assure you that you don’t need a clear lid magnifying the sun to have problems. When I first received a few slabs of Kaizen, I left one lying in the sun (temp around 70-80 degrees), and when I came back a while later, it had developed a grapefruit sized bubble. This type of foam clearly does not like sunlight or high temps.

First,thank you for the effort you put into testing…I appreciate it.
Seems like a Packout problem, while the DeWalt clear lid case got hotter,but nowhere near the packout case.
As far as I know, DeWalt doesn’t sell cases with foam or black inserts or black cups inside their cases.
Why, well maybe that’s why…it gets too hot….I have never seen or heard of a DeWalt case or storage container melting items inside…I have every case , container,tstak,tough system, Stanley case or box they have …some with foam inserts.
They are all left outside on the job all day..never a problem with heat….

Yes, it would, but Google sheets is not very nice about letting you choose your scales. Stuart was complaining to me about it last night.

Stuart’s looking into some better graphing software…You know I subscribe to MS office (begrudgingly) for my wife’s use, and it never occurred to me to use Excel.

If I wanted to highlight the difference between Dewalt and Milwaukee I could have created a graph comparing them .The point isn’t really about Dewalt vs Milwaukee though. This was more I choose the brand that was easier for me to use for most of the testing, but I didn’t want make it look like I was picking on one brand.

I’m not as familiar with sheets, but there should be a way to do it. Excel is pretty straight forward in letting you adjust the scaling.

It seems like multiple commenters glanced at the two charts, saw that Milwaukee was physically graphed higher and concluded that the packout had higher temperatures than the DeWalt, even though the opposite was what you measured.

As someone who works with students on data analysis best practices, I guess it’s a little bit of a pet peeve.

Does the Packout lid magnify the sun more than the Dewalt? I did see the test where the lid is only used as a cover. However it could be that the magnification isn’t very strong so it doesn’t show much of an effect when the material is not boxed in (ie the air was able to cool the foam before it come a problem). In a greenhouse environment the effect is much more noticeable.

I also wonder how would the Dewalt react with and without the bin with the foam insert. What happened if the Packout is partitioned similiar to the Dewalt?

Neither create a lensing effect, at least not to any significant degree. Ben observed that the transparency of the lid is important, with frosted sections diffusing and reducing the intensity of light as it passes through.

I believe that the case contents is going to be the biggest factor, with darker and matte-textured foam absorbing more light that is converted to thermal energy.

Your questions are good ones, and it’ll take a little more equipment and planning before either of us can properly answer you. We wanted to see how much interest there would be before pressing forward with additional efforts and expenses.

Given that the DeWalt recorded hotter temperatures than the Milwaukee, why would the packout have more lensing effect?

The scales are set differently in the two charts so it appears that the packout got hotter, but if you read the scales, the packout topped out at just above 140° without the foam, whereas the DeWalt was sightly hotter and still warming when the test was discontinued.

AW, it doesn’t. I will admit I was reading it wrong. To be fair I did look at the chart twice but after reading the comments I guess that was enough to create a perception blind spot. Regardless I was sloppy and that’s my fault.

Your suggestion is spot on that it would be much easier if both chart is using the same scale or better yet if two set of data is showed on the same chart. I understand that this is not a Packout vs Toughsystem post. My questions is not about that either. I am just curious about this two particular case: one red, one black and yellow with divider. Personally I would guess that red or yellow (the color) share similar heat retention property.

Both charts showed some temperate fluctuations. I wonder if the test was conducted in parallel. Were there some cloud or a cooling breeze that created the drop in temperate inside the cases before they raise back up?

I wouldn’t say that you were sloppy. You saw two charts about products you want to objectively compare, and didn’t realize that the scaling was 10° warmer on one product, which made it appear deceptively cooler than its competitor. Compounding the issue, the Milwaukee had more of a temperature difference when upside down, which makes the warmer temperature seem more extreme. I think most people glancing at the two charts would conclude that this is more of a red problem than a yellow problem.

The real issue seems to be that Google sheets doesn’t allow uses to set the scaling unless you know the back end code.

Given that the tests were probably not conducted in parallel, and therefore the results should be used for overall trend information rather than a direct comparison, I can see why they chose not to chart the results on the same graph.

I’m more concerned about batteries than I am about the foam. What effect does exposure to such high temps do to the lifecycle of a typical battery? Are some cells more resilient than others? At what temp do the cells start to fail completely and risk catastrophic damage through melting or even explosion? Don’t put those lab coats away just yet boys…

At the least, higher temperatures can cause performance or charging issues. You can’t charge hot packs, for example. I’ve had this happen, where I accidentally left a tool in the sun for 10 minutes after using it, and I had to wait before the charger would start recharging it, due to the elevated temp.

It would take a lot of energy to cook a battery pack to the point of spontaneous damage. But damage over time? Inconvenient consequences? It’s definitely possible.

A magnifying glass is a convex lens that, through refraction, focuses light rays so as to increase the intensity at the focal point. Look at the very first image. That shows uniform deformation of the foam, except where incident sunlight was blocked from reaching the foam.

Ben also saw a slight reduction of deformation at areas under the frosted plastic. That’s because of some scattering and reflecting of the light at the translucent sections of the lid.

IF there was a significant magnification or lensing effect, we’d see a pinpoint hot spot, surrounded by a radius where the foam deformation decreased proportionally with distance away from the hot spot.

I use them to keep my food warm so I’ll have a hot lunch. Although I haven’t tried a hot pocket. You might be able to boil water in one if you live in Arizona. What’s the big deal about some poorly made foam melting in a clear box sitting in the sun? What do you think is going to happen if you leave something with a clear lid sitting outside on a hot summer day? It doesn’t take a scientist to figure it out. Get a grout sponge and cut it to fit if you can’t live without your foam and be sure to get a wire brush and clean that melted shit off your tools if you were dumb enough to buy one of these and leave it out

For good measure. Today i bbqd some corn on the cobs and some peppers. Bbq was low on charcoal. Temps never hit 250. Was close to 200 the whole time. My corn and red peppers were excellent. These packout boxes are portable summertime sandwich cookers

If the lid is made of polycarbonate, it’s likely blocking UV spectrum anyway.

The problem here is that the lid is very clear. Visible light spectrum goes in, converts to thermal energy when absorbed by all materials, especially darker ones, and then that thermal energy is trapped inside the case.

What goes in, but doesn’t come out, will essentially cook the contents of the tool box.

Perhaps I missed it in the article, but what do the manufacturers of these products say about these issues?

Are there industry/company guidelines on what is an acceptable melting point for products like these? What testing do manufacturers use to ensure that a melted mess is the worst possible outcome of excessive heat?

i doubt that the melted blob could lead to flames, but in the real world it might be ‘fumes’ that are the biggest threat. If you end up with a rare cancer 20 years from now might it be possible to link that cancer back to the unexpected high end tool box melting pot from the Summer of 2018?

Ben was testing using a 3rd party foam product that users have been using inside tool boxes and organizers. Manufacturer might not have anticipated that the foam would be used in this way, but they also offer advice on how to cut finger holes in the foam via melting. (https://www.fastcap.com/product/kaizen-foam-hot-knife). The foam in my case was custom-cut for private users, and also held up with no odors or visible melting or heat-influenced deformation.

So just want to clarify part of your theory otherwise I think you’re onto something and perhaps they should consider a formula revision. Curious if the Dewalt and other brand clear boxes have the same issues the packout stuff has.

Anyway you say transparent to visible light (and I agree likely UV as well) but Opaque to IR. And you have a FLIR pic to show. I would argue it’s not Opaque but perhaps partially maybe say 40% or so. Note in your picture while it does scatter the Red hot zone of your hand – in the LH pic. There are still yellow hot spots where your fingertips are. Which I suspect were on the lid in that pic. Meaning that IR does come through it. Now maybe not 100% but it does have some scatter.

That said I would alter your theory at touch and say that it does allow IR into the box along with other light and subsequently heat – otherwise it wouldn’t get much hotter than ambient. AND I would also agree that sealed box and some bounce back causes the hot house effect to amplify in the box.

Yes open cell light foam plastics will deform/melt at temps below 200 and I wonder if they also have issues with UV exposure.

What you’re seeing there is heat from Ben’s finger tips traveling to the lid via conduction. You’re not seeing the heat from Ben’s finger tips in that image, you’re seeing the raised temperature of the lid resulting from contact with Ben’s hotter finger tips.

The imprint increases with time, and remains when the hand is removed.

So… don’t leave your tools out in the sun in a clear-top case. Not a problem! But how about expanding the discussion for when you HAVE to leave your tools out in the sun? Specifically, battery-powered tools. Sometimes you don’t have the luxury of a roofed shelter or vehicle/trailer close by.